Effect of Oligomer Structure on the Diffraction Efficiency of Holographic Recording Media

Abstract

The development of new materials possessing photoconductance in the visible and near-IR regions holds great importance for the use of such materials in photoelectric solar energy converters, light emitters, light beam modulators and switches, and devices for recording, storage, and for the treatment of optical information. In addition to inorganic semiconductors, organic oligomers and their derived composites (OC) hold promise for practical application [1-8]. Most of the experience in the development of OC with hole-type conductance has been accumulated for electrographic and holographic information media [9-11]. These OC were tested in holographic recording media (HRM) for the photothermoplastic (PTP) method of recording optical information [12]. HRM for the PTP method of hologram recording should possess suitable rheological properties, high electrical resistance in the dark, and high photoconductance. Our studies over many years have indicated that the rheological properties are more important than photoconductance in the photosensitivity of the recoding layer of photothermoplastic material. Thus, the viscosity, surface tension, brittle temperature, and viscous flow activation energy are the major factors determining the photosensitivity of the oligomer films. The softening temperature and viscosity of oligomer photoconductors decrease with decreasing length of its main chain and increasing distance between bulky carbazolyl substituents. The presence of oxygen or silicon hinge atoms enhances the number of allowed conformations of the individual units, which permits lowering the softening temperature of the oligomer and increasing elasticity of the oligomer films. However, there is only sparse information in the literature on the effect of the shape oftheoligomermoleculesontheaboveproperties.Inthiswork,wecarriedoutacomparativestudyofthediffractionefficiency of holograms in HRM derived from linear and radial oligomers. The sensitivity of the recording layer of a photothermoplastic material is largely a function of the capacity of the oligomer photoconductor to undergo deformation upon the action of a surface charge. Since one of the factors inhibiting deformation is viscosity in the melt, a decrease in this viscosity should lead to a significant increase in the sensitivity of the recording layer. The distinguishing feature of radial, i.e., star-shaped, polymers is lower viscosity in melts and solutions relative to their linear analogs. This property led to the choice of radial siloxane oligomers for the creation of highly-sensitive HRM for the PTP method of hologram recording.